1. Field of the Invention
The present invention relates to an electro-pyrotechnic initiator. More specifically, the present invention relates to a device that utilizes a foil resistive element to generate heat and utilizes the heat produced to achieve autoignition temperature in an energetic or pyrotechnic material.
2. Problems in the Art
Electro-pyrotechnic initiators are known in the art. These types of initiators, which are used in variety of settings such as military and air bag applications, suffer from a variety of deficiencies. For example, prior art initiators either require a relatively large amount of energy to initiate autoignition or become more costly with less reliability when reduced to low energy activation levels. In this specification, autoignition temperature is the temperature at which a sample either deflagrates or detonates (in an energetic material), or rapidly combusts or decomposes to release gas, heat, or light (in a pyrotechnic material), under contact heating conditions or by heat of sublimation generated by the heating element. In one such prior art device, known as bridgewire technology, reducing the energy level requires using an electrically conductive wire of such narrow diameter that variations in contact point result in large variations in resistance; parts are more difficult and costly to produce; and the wire elements are more susceptive to damage, thereby reducing reliability. In addition, prior art initiators often utilize multiple levels of successively more sensitive energetic or pyrotechnic materials, adding to the overall cost of the initiator. Finally, variance in the resistivity of the resistive element from the beginning to the end of the manufacturing process leads to unreliability in prior art initiators.
One commonly used method for implementing an electro-pyrotechnic initiator is known as the bridgewire technology, wherein the bridgewire is welded between two contact points. An energetic or pyrotechnic material is compacted against the wire using pressures reaching in excess of 10,000 pounds per square inch. The wire is stretched flat against the very uniform coplanar surfaces of the header, the glass insulator, and the top of the pin, to support the very delicate wire against the high compression forces of the compacted material. Therefore, the bridgewire technology requires the header to be lapped or ground flat. The grinding of the header adds cost to the electro-pyrotechnic initiator. In addition, the bridgewire technology requires costly equipment to orient the product to achieve the precise welding positions that are necessary to get the correct resistance value. Furthermore, the bridgewire technology has two zones of potential electrostatic discharge that could either produce an electric arc or make an electrical contact which would reduce the effective resistance of the bridgewire, making it responsive to an activation energy lower than the designed autoignition energy which would result in accidental firing of the device at electrostatic discharge energies lower than their designed safety levels. The accidental firing of the initiator would expose vehicle occupants or mechanics to unnecessary danger.
To increase the safety of automobiles, automobile manufacturers are increasing the number of air bags within the vehicle. The proliferation of air bags increases the energy reserve requirements needed to fire the air bags. The energy reserve is normally stored as voltage on charged capacitors. The energy storage capacity is directly proportional to the electrical capacitance of the capacitor. Occupant safety systems on contemporary vehicles require relatively high capacitance, which adds to both size and cost. In general, electrolytic capacitors are used for energy storage in contemporary systems. Lower cost ceramic capacitors do not have the capability to store the relatively high energy required to activate contemporary systems. Therefore, a lower energy initiator is needed to reduce the total activation energy reserve needed for the system.
The addition of multiple autoignition-deployed safety devices such as airbags, seatbelt pretensioners, battery cable disconnects, fuel line shut off devices, roll bars, etc. in automobiles also requires the use of "smart" air bag systems. These smart air bag systems use an application specific integrated circuit (ASIC) to receive control signals and control the firing of the initiator. Much of the area of the ASIC is devoted to handling current, while only a small part of the area of the ASIC is devoted to the control signals. The energy requirements of the prior art devices require the ASIC to have a larger size to handle the current requirements. A lower energy device would decrease the size of the ASIC and decrease the overall cost of the system.
An electro-pyrotechnic initiator is generally disclosed in U.S. Pat. No. 5,544,585. This device discloses a foil resistive element to which a thermosensitive substance is coated in the form of an explosive varnish. The heat generated by the resistive element ignites the explosive varnish which in turn fire a primary charge. Adding an explosive varnish to the resistive element increases the cost of the initiator.
In addition, the initiator disclosed in U.S. Pat. No. 5,544,585 requires a relatively large amount of energy in order to ignite the explosive varnish. To fire this initiator using a capacitor, a relatively expensive electrolytic capacitor must be used in order to obtain the capacitance needed to store enough energy. The electro-pyrotechnic initiator of the present invention operates in a lower energy area and does not require the use of an explosive varnish in order to detonate a primary charge. Due to its low energy requirements, a cheaper ceramic capacitor can be used to fire the electro-pyrotechnic initiator of the present invention. In addition, the electro-pyrotechnic initiator of the present invention, for the same input, can achieve higher temperatures than the initiator disclosed in U.S. Pat. No. 5,544,585. In other words, the initiator of the present invention requires less energy to achieve the same temperature response obtainable by the prior art initiator.